1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device that is able to prevent defects in a driving circuit and a method of fabricating the same.
2. Description of the Related Art
Generally, a liquid crystal display (LCD) controls the light transmissivity of liquid crystal cells arranged in a matrix in accordance with video signals to display a picture that corresponds to those video signals. A thin film transistor (TFT) is generally used as a device that switches the liquid crystal cells.
The thin film transistor used in the liquid crystal display device has its semiconductor layer made of amorphous silicon or poly silicon. The amorphous silicon type thin film transistor has an advantage that its characteristic because of the relatively high uniformity of the amorphous silicon film. However, the amorphous silicon type thin film transistor has a disadvantage that its response speed is slow because of low electric charge mobility. Accordingly, the amorphous silicon type thin film transistor has a disadvantage that it is not applicable as the driving device of a gate driver and a data driver of a high resolution display panel that requires rapid response speed.
The poly silicon type thin film transistor has high electric charge mobility so that it can have surrounding driving allowing it to be built into the display panel as well as being applicable to a high resolution display panel. Accordingly, the liquid crystal display device using the poly silicon type thin film transistor are becoming popular.
FIG. 1 is a plan view representing a liquid crystal display device using a poly silicon type thin film transistor according to the related art.
Referring to FIG. 1, a related art liquid crystal display device using a poly silicon type thin film transistor includes a picture display part 96 having a pixel matrix, a data driver 92 to drive data lines 4 of the picture display part 96, and a gate driver 92 to driver gate lines 2 of the picture display part 96.
The picture display part 96 has liquid crystal cells LC arranged in a matrix shape to display a picture. Each liquid crystal cell LC is driven by a thin film transistor TFT 30 switching device connected to a crossing of the gate line 2 and the data line 4, made of a poly silicon into which N-type impurities are injected.
The N-type TFT 30 responds to a scan pulse from the gate line 2 to charge the liquid crystal cell LC with a video signal, i.e., a pixel signal, from the data line 4. Accordingly, the liquid crystal cell LC controls the light transmissivity in accordance with the pixel signal with which the liquid crystal cell LC is charged.
The gate driver 94 sequentially drives the gate lines 2 for a horizontal period each frame by gate control signals. The gate driver 94 sequentially turns on the thin film transistors by the horizontal line to connect the data line 4 with the liquid crystal cell.
The data driver 92 takes samples of a plurality of digital data signals by the horizontal period to convert it into an analog data signal. And, the data driver 92 supplies the analog data signals to the data lines 4. Accordingly, the liquid crystal cells connected to the turned-on thin film transistor respond to the data signals from the data lines 4 to control the light transmissivity.
The gate driver 94 and the data driver 92 include a driving device connected in a CMOS structure. The driving device is a single large TFT that has a wide channel with width Wa in order to make a large amount of electric current flow for switching a relatively high voltage. The poly-silicon is used for such a driving device to obtain a rapid response.
FIG. 2 is a plan view representing a driving device of a driving circuit part according to the related art. FIG. 3 is a sectional view representing the driving device shown in FIG. 2.
Referring to FIGS. 2 and 3, a driving device of a driving circuit part having one thin film transistor includes an active layer 74 in which an purity, i.e., N+ ion or P+ ion, is injected formed on a lower substrate 20; a gate electrode 66 formed to overlap a channel area 74C of the active layer 74 with a gate insulating film 42 therebetween; source/drain electrodes 68, 70 formed to be insulated having the gate electrode 66 and an intermediate insulating film 56 therebetween; and a protective film formed on the source/drain electrodes 68, 70.
Each of the source/drain electrodes 68, 70 are in contact with the source/drain area 74S, 74D of the active layer 74 into which a designated impurity is injected through source/drain contract holes 84S, 84D that penetrate the gate insulating film 42 and the intermediate insulating film 56. The protective film 48 is formed on the source/drain electrodes 68/70 to play the role of protecting the driving device.
On the other hand, the driving device composed of one thin film transistor of the related art in this way has an advantage in that a lot of electric current can flow. However, a lot of heat is generated at the channel 57 due to the flow of electric current. Accordingly, a structure that can cool down the heat generated at the channel 57 is illustrated in FIG. 4, in which the driving device of the thin film transistors has a plurality of narrow channel widths Wb that are connected in parallel.
The driving device of the driving circuit part illustrated in FIG. 4 has a structure that the thin film transistors of a plurality of channels 77 are connected in parallel so that the sum of the channel widths Wb is the same as the width Wa of the one channel shown in FIG. 2 (small channel width (Wb)×the number of channels (n)=width of one channel (Wa)). Between the thin film transistors, there exist the gate insulating film 42 and the intermediate insulating film 56 between the two channels 77 as illustrated in FIG. 5. The heat generated at the channel 77 is transmitted by the gate insulating film 42 and the intermediate insulating film 56, and then it is emitted to the outside of the driving device.
However, the channel width Wb and the distance between channels D are uniform, so the amount of heat generated in the channel widths Wb is the same, but the heat emission is not greater in the channel 77 located at the central part of the driving device than in the channel 77 located at the edge, thus, as shown in FIG. 6, the heat distribution A is relatively higher in the channel 77 area of the central part of the driving device than in the channel 77 area of the edge. In other words, the amount of the gate insulating film 42 and the intermediate insulating film 56, which the heat is transmitted through and emitted from, in the edge of the driving device is greater than in the center thereof, thus further in from the edge of the driving device to the center, the smaller the heat emission capacity of the heat generated at the channel becomes.
Accordingly, the emission of the heat is worse in the channel 77 of the central part of the driving device of the driving circuit part than in the channel 77 at the edge thereof, thus the central part of the driving device deteriorates as illustrated in FIG. 7. As a result, driving device defects occur, e.g., the smooth flow of electric current is interfered or the characteristic of the driving device deteriorates. In general problems occur in which the driving device is not driven in a normal way.
Further, the LCD device has a structure in which the thin film transistor with a plurality of channel 77 are connected in parallel in order that the sum of the channel widths Wb is the same as the width Wa of one channel shown in FIG. 2 {small channel width (Wb)×the number of channels (n)=width of one channel (Wa)}. Between the thin film transistors, there are [the] gate insulating film 42 and the intermediate insulating film 56 between each of the channels 77 as illustrated in FIG. 5, so the heat generated at the channel 77 is absorbed by the gate insulating film 42 and the intermediate insulating film 56. The material of the gate insulating film 42 and the intermediate insulating film 56 is an insulating material such as SiO2 that has a low dielectric constant. This reduces the value of the parasitic capacitance generated between electrodes. But the insulating material, such as SiO2, has for example low heat conductivity of about 1.4 kW/mK. Accordingly, only part of the heat generated at the channel 77 is transmitted to the gate insulating film 42 and the intermediate insulating film 56; the remaining heat is not transmitted. As a result, the heat remaining at the channel 77 causes the channel to deteriorate. Thus, the flow of electric current degrades or the characteristic of the driving device deteriorates resulting in the defect of the driving device, thereby generating a problem that the driving device is not driven in a normal way.